Iridium, like other elements of Group VIII of the Periodic Table, is known to be active for hydrogenation/dehydrogenation reactions and, in particular, for the carbon-carbon bond scission reactions involved in ring opening of cyclic hydrocarbons. For example, U.S. Pat. No. 7,579,511 lists iridium as one suitable Group VIII metal for catalyzing the ring opening conversion of methylcyclopentane produced as a by-product of the hydroalkylation of benzene to cyclohexylbenzene.
In view of its scarcity and high cost, any commercial application of iridium as a catalyst requires that utilization of the iridium is maximized, or in other words that the iridium is effectively and uniformly dispersed on its catalytic support in a thermally stable manner. However, few studies have focused on producing highly dispersed, stable supported iridium catalysts, particularly on non-acidic supports such as silica.
For example, US Published Patent Application No. 2006/0166809 discloses a process for the manufacture of a catalyst comprising a catalytically active metal dispersed on a support, which process comprises a) treating a porous support with a compound or salt of the metal and a organic compound selected from (i) amino acids and (ii) compounds containing both an amino group and an alcohol group, to form the organic metal complex on the support; b) partially decomposing the organic metal complex on the support to the extent that the partially decomposed product (I) retains between 10 and 95% by weight of the dry weight attributed to the organic complex prior to partial decomposition, and (II) exhibits one or more infra-red absorption bands between 2100-2200 cm−1 that are not present in the organic complex before partial decomposition; and converting the partially decomposed organic metal complex into catalytically active metal. However, although the '809 application lists iridium as one of about forty suitable catalytically active metals, the Examples focus on the production of ruthenium catalysts via reaction of a nitrate salt. In contrast, no nitrate salt is available with iridium.
According to the present invention, it has now been found that a silica-supported iridium catalyst with excellent dispersion and nanoscale homogeneity can be prepared by modifying the partial decomposition process described in the '809 application. The resultant iridium catalyst exhibits higher hydrogen chemisorption uptakes and activity for carbon-carbon bond hydrogenolysis than conventional iridium catalysts.